Methods to slice a silicon ingot

ABSTRACT

The present disclosure generally relates to methods for recovering silicon from saw kerf, or an exhausted abrasive slurry, resulting from the cutting of a silicon ingot, such as a single crystal or polycrystalline silicon ingot. More particularly, the present disclosure relates to methods for isolating and purifying silicon from saw kerf or the exhausted slurry, such that the resulting silicon may be used as a raw material, such as a solar grade silicon raw material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.12/647,991, filed Dec. 28, 2009, which claims the benefit of U.S.Provisional Patent Application Ser. No. 61/141,974, filed Dec. 31, 2008,the entire contents of which are incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to methods for recoveringsilicon from saw kerf, or an exhausted abrasive slurry, resulting fromthe cutting of a silicon ingot, such as a single crystal orpolycrystalline silicon ingot. More particularly, the present disclosurerelates to methods for isolating and purifying silicon from saw kerf orthe exhausted slurry, such that the resulting silicon may be used as araw material, such as a solar grade silicon raw material.

BACKGROUND OF THE DISCLOSURE

Silicon wafers are conventionally prepared from a single crystal or apolycrystalline silicon ingot, which typically has a cylindrical shape.The ingot is sliced in a direction normal to its longitudinal axis toproduce as many as several hundred thin, disk-shaped wafers. The slicingoperation is typically accomplished by means of one or morereciprocating wire saws, the ingot being contacted with thereciprocating wire while a liquid slurry containing abrasive grains,such as silicon carbide, is supplied to a contact area between the ingotand the wire. Conventional wire saw slurries typically comprise alubricant acting as a suspending and cooling fluid, such as, forexample, a mineral oil or some water-soluble liquid (e.g., polyethyleneglycol, or PEG).

As the ingot is sliced, the abrasive particles of the slurry are rubbedby the wire saw against the ingot surface, causing silicon particlesfrom the ingot to be removed, as well as metal (e.g., iron) from thewire itself. A significant amount of silicon particles are lost duringcutting. The silicon material that accumulates as an ingot is sliced isconventionally known as the “saw kerf.” As the concentration of silicon,as well as other particulate (e.g., metal particulate), in the slurryincreases, the efficiency of the slicing operation decreases.Eventually, the slurry becomes ineffective, or “exhausted,” and then itis typically disposed of or discarded. Traditionally, the exhaustedslurry has been disposed of by incineration or treated by a waste watertreatment facility. However, burning the slurry generates carbon dioxideand sending the slurry to a waste water treatment facility typicallyresults in the formation of a sludge that must be disposed of in alandfill. Accordingly, both approaches of disposal are unfavorable froman environmental point of view, as well as the costs associatedtherewith. As a result, some have proposed methods by which the abrasiveslurry can be recycled and reused. (See, e.g., U.S. Pat. No. 7,223,344,the entire contents of which are incorporated herein by reference forall relevant and consistent purposes.)

However, in addition to the environmental and economic concernsassociates with the saw kerf, or exhausted slurry, the loss ofpotentially useful silicon material should also be considered.Specifically, although wire saw technology has improved, each pass ofthe wire through the silicon ingot results in the lost of an amount ofsilicon equivalent to about a 250 to 280 micron thick slice of theingot. As technology enables thinner and thinner wafers to be slice fromthe ingot, more and more passes of the wire through the ingot occurs,resulting in more and more loss of silicon to saw kerf. For example,with existing wire saw technologies, kerf loss can represent from about25% to about 50% of the silicon ingot material.

While there have been some general suggestions of recovering the siliconmaterial from the saw kerf or exhausted slurry for use in, for example,photovoltaic cells (see, e.g., U.S. Pat. No. 6,780,665, the entirecontents of which is incorporated herein by reference for all relevantand consistent purposes), there are several drawbacks. For example, thepreviously known methods do not provide a means of addressing bulk andsurface metal contamination that may be present in silicon obtained fromthe kerf. This can have a significant impact on the purity of therecovered silicon, and subsequently on the end uses that are availablefor the recovered silicon. Further, the methods of silicon recovery(e.g., froth flotation recovery) used to-date typically do not recoveran adequate amount of silicon particles for re-use.

Accordingly, there remains a need for a method to recover and purifysilicon particles created by the cutting of silicon ingots so that therecovered silicon can, for example, be melted and recycled for use invarious applications, including solar grade silicon material.Optionally, such a method would additionally allow for the recovery ofthe silicon carbide used in the slurry process, so that it may also bere-used.

SUMMARY OF THE DISCLOSURE

Briefly, therefore, the present disclosure is directed to, in oneembodiment, a method for separating and recovering silicon particlesfrom silicon saw kerf resulting from slicing silicon wafers from asilicon ingot, the saw kerf comprising a lubricating fluid and a mixtureof solid particulate comprising abrasive grains, silicon particles,metal particles and oxide particles. The method comprises: separating atleast a portion of the lubricating fluid from the solid particulatemixture; washing the solid particulate mixture with an acidic solutionin which (i) the silicon particles are substantially insoluble, and (ii)the metal particles and oxides particles are soluble, metal particlesand oxide particles being dissolved from the solid particulate mixture;collecting the washed solid particulate mixture, the washed mixturecomprising silicon particles and abrasive grains; and, separating thesilicon particles from the abrasive grains in the washed solidparticulate mixture. Advantageously, the separated silicon particles mayhave a carbon content of less than about 50 ppma and have a content ofmetal contaminants of less than about 150 ppma.

The present disclosure is further directed to such a method whereinwashing the solid particulate mixture comprises contacting the solidparticular mixture with an acidic solution capable of creating aflotation froth with the solid particulate mixture, the froth comprisingsilicon particles and abrasive grains.

The present disclosure is further directed, in another embodiment, to amethod for separating and recovering silicon particles from silicon sawkerf resulting from slicing silicon wafers from a silicon ingot, the sawkerf comprising an organic lubricating fluid and a mixture of solidparticulate comprising abrasive grains, silicon particles, metalparticles and oxide particles. The method comprises: contacting the sawkerf with a chelating agent soluble in the organic lubricating fluid toform a complex with one or more metals present in the saw kerf; mixingthe chelated saw kerf solution with an aqueous acid solution andallowing the mixture to separate into an aqueous phase and an organicphase, the aqueous phase comprising silicon particles and the organicphase comprising the complex formed between chelating agent and themetals; collecting the aqueous phase comprising the silicon particles;and, recovering at least a portion of the silicon particles from theaqueous phase. Advantageously, the recovered silicon particles may havea carbon content of less than about 50 ppma and have a content of metalcontaminants of less than about 150 ppma.

The present disclosure is further directed to one or more of thepreceding embodiments, wherein the silicon particles are recovered bysubjecting the (i) collected, washed solid particulate mixture, or (ii)the froth formed from the washed solid particulate mixture, or (iii) theaqueous phase comprising the silicon particles, to a density-dependentseparation technique, and in particular a density-dependent separationtechnique selected from sedimentation centrifugation, filtrationcentrifugation, and hydro-cyclone separation.

The present disclosure is further directed to one or more of thepreceding embodiments, wherein the silicon particles are recovered byfirst drying the (i) collected, washed solid particulate mixture, or(ii) the froth formed from the washed solid particulate mixture, or(iii) the aqueous phase comprising the silicon particles, and thensubjecting the dried froth to a non-uniform magnetic field (i.e., amagnetic field gradient) in order to separate the silicon particles fromthe abrasive particles, and more particularly silicon carbide abrasiveparticles.

The present disclosure is still further directed to one or more of thepreceding embodiments, wherein the abrasive grains, and in particularsilicon carbide, are additionally recovered for re-use.

The present disclosure is further directed to a method for preparing asolar grade silicon pellet from a silicon saw kerf as detailed above.The method comprises: (i) recovering silicon particles from the saw kerfby one of the preceding embodiments; (ii) melting the recovered silicon;and, (iii) forming a solar grade silicon pellet from the melted silicon.

The present disclosure is further directed to a method for slicing asilicon ingot. The method comprises contacting a surface of the siliconingot with a reciprocating wire saw and a slurry comprising an organiclubricating fluid, an abrasive particulate, and a metal chelating agentsoluble in the organic lubricating fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a workflow depicting a general overview of the process of thepresent disclosure, including separation of the lubricating agent fromthe solid particulate mixture in the saw kerf.

FIG. 2 is a workflow depicting a general overview of a portion of theprocess of the present disclosure, and in particular the options forremoving metal and oxide particles from the solid particulate mixture.

FIG. 3 is a workflow depicting a general overview of a portion of theprocess of the present disclosure, and in particular an optional methodfor removing metal and oxide particles from the saw kerf and solidparticulate mixture.

FIG. 4 is a workflow depicting a general overview of a portion of theprocess of the present disclosure, and in particular the options forseparating silicon particles from the abrasive grains (e.g., siliconcarbide).

FIG. 5 depicts an arrangement of 12 40 Moe magnets with an iron wrapperand iron poles in the center. The magnetic field lines are made tofollow the cross-section of a toroid around the air core. Thearrangement is 12 mm². The arrows in each magnet indicate magneticnorth.

FIG. 6 depicts a magnitude of |H| relative to the iron poles and therespective density plots of |H| and A/m².

FIG. 7 depicts HgradH across the center of the magnetic separator. Thesolid line or curve represents the numeric calculation and the broken ordashed line or curve represents the numeric expectation.

FIG. 8 depicts the terminal velocities by particle radius of siliconcarbide, silicon, and silicon dioxide, where HgradH=5×10¹⁵ A²/m³ andsame sign as g. The units of measurement are mm/minute for particles inwater.

FIG. 9 is a graphical depiction of the magneto-Archimedes effect whereinthe magnetic configuration of FIGS. 5 and 6 is rotated.

FIG. 10 depicts the terminal velocities by particle radius of siliconcarbide, silicon and silicon dioxide in a field gradient whereHgradH=2×10¹⁶ A²/m³ and same sign as g. The units of measurement aremm/minute for particles in the air.

FIG. 11 depicts a schematic illustration of an inclined magneticseparator.

FIG. 12 depicts the terminal separation velocities of silicon carbide,silicon and silicon dioxide versus particle diameter for siliconcarbide, silicon and silicon dioxide by gravity and magnetic filter inliquid air where HgradH=0.037·10¹⁶ A²/m³.

FIG. 13 depicts aqueous sodium metatungstate viscosity versus density.

FIG. 14 depicts the centrifugal density separation at 17,000 g in sodiummetatungstate liquid. FIG. 14 further depicts the sedimentation velocityof 500 nm silicon and 100 nm silicon carbide particles versus sodiummetatungstate density.

FIG. 15A depicts the time dependant bulk metal out-diffusion from asphere for initial concentration u=1, initial total content in reducedunits is 4/3Π when a=1 and D=1. FIG. 15B depicts radial concentrationprofiles at reduced times 0.0001, 0.01, 0.04, 0.08, 0.16, and 0.32.

FIGS. 16A and 16B depict images of silicon kerf particles separated fromsaw kerf.

FIGS. 17A-17D depict EDX spectra of four different silicon particles.

FIG. 18A is a 400 micron Nomarksii microscope image of flakes broken offof a silicon pellet. FIGS. 18B and 18C depict 100 micron Nomarksiimicroscope images of flakes broken off a silicon pellet, showinginterior and exterior surfaces. FIG. 18D is a 3 to 10 micron Nomarksiimicroscope image of flakes broken off of a silicon pellet.

It is to be noted that corresponding reference characters indicatecorresponding parts throughout the several views of the drawings.

It is to be further noted that the design or configuration of thecomponents presented in these figures are not to scale, and/or areintended for purposes of illustration only. Accordingly, the design orconfiguration of the components may be other than herein describedwithout departing from the intended scope of the present disclosure.These figures should therefore not be viewed in a limiting sense.

DETAILED DESCRIPTION OF THE DISCLOSURE

In accordance with the present disclosure, it has been discovered thatsilicon particles in a silicon ingot saw kerf (or an exhausted slurry)resulting from the slicing of a silicon ingot) can be efficientlyisolated and purified, thus rendering the isolated and purifiedparticles suitable for reuse as a raw material in other siliconapplications (e.g., solar grade silicon). As further detailed hereinbelow, and as illustrated, for example, in the workflows of FIGS. 1-4,the isolation and purification of the silicon particles from the sawkerf may be achieved using a sequence of one or more generally knownliquid/solid, liquid/liquid, and/or solid/solid separation techniques,in combination with the treatment of the saw kerf, or a portion thereof,with an aqueous acid solution. The resulting silicon particles may have,for example, a carbon content of less than about 50 ppma (parts permillion atomic), about 40 ppma, about 30 ppma, about 25 ppma or less(e.g., about 20 ppma, about 15 ppma, or even about 10 ppma), and/or mayhave a total content of metal contaminants (e.g., copper, nickel, iron,etc.) of less than about 150 ppma, about 125 ppma, about 100 ppma orless (e.g., about 90 ppma, about 70 ppma, or even about 50 ppma), asfurther illustrated in the working Examples.

In this regard it is to be noted that, as used herein, the phrase “sawkerf” generally reference to the waste material that results from theslicing of sawing process utilized to cut a silicon ingot. This phrasemay optionally be used herein interchangeable with the phrase “exhaustedslurry” from a silicon ingot slicing or sawing process, which generallyrefers to a slurry that is essentially no longer suitable for purposesof slicing silicon wafers from a silicon ingot as a result of, forexample, an unacceptably high content of silicon and/or metalparticulate that hinders the slicing operation. Some believe thatsilicon particulate hinders a silicon ingot slicing operation atconcentrations above about 1-5% by weight, of the solid matter in theslurry. It is also believed that metal particulate hinders the slicingoperation at concentrations above about 0.5-2% by weight, of the solidmatter in the slurry.

Additionally, “spent abrasive grains” generally refers to abrasivegrains (e.g., silicon carbide, or SiC) that, as a result of being worndown by the slicing process, are of a diameter or size which isgenerally no longer suitable for purposes of slicing silicon wafers froma silicon ingot. Some believe that abrasive grains are spent if theyhave a particle size of less than about 1 micron (e.g., an approximatediameter of less than about 1 micron). “Unspent abrasive grains”generally refers to abrasive grains in the exhausted slurry which arestill suitable for purposes of slicing silicon wafers from a siliconingot, such grains typically have a particle size of greater than about1 micron. Some believe that the spent abrasive grains hinder a siliconingot slicing operation at concentrations above about 5-10%, by weightof the total abrasive grains (i.e., spent and unspent abrasive grains).

1. Analysis of Exemplary Saw Kerf/Exhausted Slurry

Gravimetric analysis was performed on representative samples of saw kerfwaste material slurry obtained from representative wafer slicingprocesses of single crystal silicon ingots. These waste slurry sampleswere found to include polyethylene glycol (PEG), which was used as thelubricating fluid therein, silicon carbide, which was used as theabrasive grains therein, iron, copper, zinc, silicon, their respectiveoxides, and other various impurities (of lesser concentrations ascompared to the other noted components). More specifically, bysuccessive treatments of samples (both the raw “saw waste” material, aswell as the “process waste” obtained from a commercially availableslurry recovery system) by means of rinsing with water, an aqueous HClsolution, and aqueous HF solution, an aqueous HF/NO₃ solution, withintermediate drying and weighing steps, the overall composition of therepresentative saw kerf waste samples were found to be as reported inTable 1 and Table 2, below.

TABLE 1 Mass Fraction of Saw Kerf Components Including PEG. (propagatedweighing errors are one standard deviation) Component weight fractionProcessed Waste Saw Waste Polyethylene glycol 0.438 +/− 0.001 0.320 +/−0.001 HCl removable 0.010 +/− 0.001 0.006 +/− 0.001 (metals) HFremovable 0.021 +/− 0.001 0.006 +/− 0.001 (oxides) HF + HNO₃ 0.011 +/−0.001 0.009 +/− 0.001 removable (silicon) Impervious material 0.519 +/−0.002 0.660 +/− 0.003 (SiC)

TABLE 2 Mass Fraction of Saw Kerf Components Excluding PEG. (propagatedweighing errors are one standard deviation) Solids only weight fractionProcessed Waste Saw Waste HCl removable 0.018 +/− 0.002 0.009 +/− 0.002(metals) HF removable 0.038 +/− 0.002 0.008 +/− 0.002 (oxides) HF + HNO₃0.020 +/− 0.002 0.013 +/− 0.013 removable (silicon) Impervious material0.924 +/− 0.006 0.970 +/− 0.006 (SiC)

As the results of the Tables above indicate, recovery of the siliconparticles present in the saw kerf or exhausted slurry may be achieved byproper selection of a solvent (or solvents) that dissolve unwantedcontaminants (e.g., metal particles, oxide particles, etc.), oralternatively by proper selection of a chelating agent which traps theunwanted contaminants (e.g., metal particles), or a combination thereof,without dissolving the silicon particles, and then further selecting anappropriate means for separating the solid silicon particulate from theother remain undesirable solids (e.g., abrasive grains, such as siliconcarbide particles). Accordingly, in one or more of such embodiments, thesaw kerf or exhausted slurry is contacted with a solvent in which thesilicon particles are substantially insoluble (e.g., a solvent in whichless than about 5%, about 3%, about 1%, about 0.5%, or less, by weightof the silicon present is dissolved when contacted with the solvent). Inan alternative embodiment, however, a combination of solvent and achelating agent may be used, the chelating agent trapping metalcontaminants for removal from the saw kerf or exhausted slurry (and morespecifically from the silicon particles that are to be recovered).

2. Removal of Metal/Oxide Particles and Contaminants

Referring again to FIGS. 1-4, it accordingly to be noted that thepresent disclosure is directed to methods for separating and recoveringsilicon particles from silicon saw kerf resulting from slicing siliconwafers from a silicon ingot. As previously noted above, the saw kerftypically comprises a lubricating fluid and a mixture of solidparticulate, the solid particulate including or comprising abrasivegrains (e.g., silicon carbide), silicon particles, metal particles andoxide particles. Generally, in a first embodiment the method comprisesoptionally separating at least a portion (e.g., about 25%, about 50%,about 75% or more, by weight) of the lubricating fluid from the solidparticulate mixture, and then washing the resulting solid particulatemixture (or slurry, depending upon how much, if any, of the lubricatingfluid is removed) with one or more acidic solutions in which the siliconparticles are substantially insoluble, and the metal particles, theoxides particles, or both, are soluble. In this way, the metal particlesand/or the oxide particles may be dissolved and separated from whatremains of the solid particulate mixture (and in particular the siliconparticles therein). The washed solid particulate mixture, whichcomprises silicon particles and abrasive grains (e.g., silicon carbide),may then be subject to a solid separation process of some kind in orderto separate the silicon particles from the abrasive grains.

In an alternative embodiment, such as wherein the saw kerf comprises anorganic lubricating fluid, metal contaminants may be removed therefromby means of a chelating agent that is soluble in the organic lubricatingfluid. Specifically, the saw kerf is contacted with a chelating agentsoluble in the organic lubricating fluid to form a complex with one ormore metals present in the saw kerf. The chelated saw kerf, which may bein the form of a slurry or suspension for example, may then becontacted, and optionally agitated with, an aqueous acid solution. Theresulting aqueous/organic mixture is then allowed to separate into anaqueous phase and an organic phase, the aqueous phase comprising siliconparticles and the organic phase comprising the complex formed betweenchelating agent and the metals. The two liquid phases may be separatedusing means generally known in the art. After collecting the solids fromthe aqueous phase (by filtration, evaporation/drying, etc.), theresulting solids may be separated to collect or recover at least aportion of the silicon particles from the other solids present (e.g.,the abrasive grains).

In this regard it is to be noted that the ratio of organic solution toaqueous solution, the number of liquid/liquid extractions, etc. may bedetermined experimentally, in order to optimize the amount of siliconparticles that are recovered.

It is also to be noted that various options for removing the metaland/or oxide particles from the saw kerf are further detailed hereinbelow.

A. Optional Removal of Lubricating Fluid

As noted above, substantially all, or a portion of, the lubricatingfluid present in the saw kerf, or exhausted slurry, may optionally beremoved prior to further treatment of the solid particulate mixture. Ifdesired, essentially any known method for separating solids of a smalldiameter (e.g., typically between about 0.5 microns and about 25microns, the silicon particles for example typically falling within aparticle size range of between about 0.5 microns and about 10 microns,or about 0.75 microns about 7.5 microns, while the abrasive grains, suchas silicon carbide, typically fall within a particle size range ofbetween about 2.5 and about 25 microns, or about 5 microns and about 20microns) may be used. In one preferred embodiment, the method employedis one that yields a lubricating fluid that is substantially free ofsolids (e.g., preferably less than about 1 g of solids per liter oflubricating fluid). Filtration of the saw kerf, or exhausted slurry,using techniques generally known in the art, such as press filtration,is an example of a method that may be used to separate the solid matterfrom the lubricating fluid. Press filtration generally involvesseparating the saw kerf or exhausted slurry into a liquid fraction and asolids fraction (e.g., a solid particulate mixture) by passing itthrough at least one screen, such as a polypropylene screen, having apore or mesh size which is sufficient to remove substantially all of thesolids from the fluid at an elevated pressure. (See, e.g., filtrationdetails provided in U.S. 2004/0144722, which is incorporated herein byreference.)

Filtration yields a “cake” of solids (i.e., a solid particulate mixture)that comprises the silicon particles, metal particles, and abrasivegrains (i.e., spent abrasive grains, unspent abrasive grains), andlikely at least some trace amount of the lubricating fluid (theconcentration of lubricating fluid in the cake typically being, forexample, less than about 25%, 20%, 10%, or even 5%, by weight of thecake). The solid particulate mixture resulting from the filtration orseparation may then be further processed, in order to isolate siliconparticulate from the other solid particles present therein. Optionally,however, prior to further separating the solids, and while still in thefiltering apparatus, the solids may be washed with a solvent (e.g.,water or methanol) to reduce the concentration of lubricating fluidpresent therein.

B. HCl/HF Solutions, and/or Chelating Agents

In one embodiment, the saw kerf may be contacted (e.g., rinsed orwashed) with individual aqueous solutions of hydrochloric acid (HCl) andhydrofluoric acid (HF), in order to dissolve and remove metal particlesor contaminants (HCl solution), and oxide particles or contaminants (HFsolution) present therein, and/or to remove metal contaminants presenton or in (e.g., present in the bulk of) the silicon particles.

With respect to the removal of metal or oxide particles, it is to benoted that the concentration of the acid solutions, and/or the number oftimes the saw kerf is contacted with each one, and/or the amount of acidsolution needed for a given amount of saw kerf, may be optimized for agiven saw kerf in order to maximize contaminant removal. Typically,however, the saw kerf may be treated with between about 1 and about 5,or about 2 and about 4, stoichiometric equivalents of the aqueous HFsolution, the HCl solution, or both, relative to the concentration (ornumber of equivalents) of silicon present therein. Additionally, oralternatively, a typically suitable aqueous HCl solution has a HClconcentration of about 0.5 to about 0.25 normal (N), or about 0.75 toabout 0.2 N, or about 0.1 normal (N), while a typically suitable aqueousHF solution has a HF concentration of about 40% to about 60% (byweight), or about 45% to about 55%, or even about 49%.

In this regard it is to be noted that, in addition to dissolving metaland oxide contaminants or particles, treatment of the saw kerf with theacidic solutions may also act to break bonds present between siliconand, for example, carbide, that may be present, thus enabling separationof the silicon from the carbide.

In this regard it is to be further noted that, when certain fastdiffusing metals (i.e., metals, such as copper or nickel, capable ofdiffusing or moving throughout the silicon in a relative short period oftime, on the order of a few hours—e.g., less than about 8 hours, about 6hours, about 4 hours, or even about 2 hours, at for example roomtemperature) are present, the saw kerf, and more specifically thesilicon particles present therein, may be “aged” or allowed to stand atan appropriate temperature (e.g., room temperature) for a period oftime, either before or during the acid treatment (i.e., washing orrinsing), to allow time for the diffusing metals to reach the surface ofthe silicon particle and be trapped and/or removed by the acid solution.

In this regard it is to be still further noted that in saw kerfcontaining polyethylene glycol (PEG), the metals may be removed orleached from the silicon particles by repeated washing with water andthe aqueous acid solution. In the presence of HF and essentially nosurface oxide, copper will plate out onto the silicon particle surface.In the presence of HCl, however, copper deposition on the surface of thesilicon particles can be prevented. Therefore, in aqueous systems (i.e.,saw kerfs or slurries), at least one cycle or step of the washing orleaching step will preferably not use HF, but rather will preferably useHCl.

In this regard it is to be still further noted that, in an alternativeembodiment, a chelating agent may be used alone or in combination (e.g.,sequentially) with one or more of the aqueous acidic solutions. Achelating agent may be use to trap or sequester the metal as it diffusesto the surface of the silicon particles. Essentially any known chelatingagent that is effective for sequestering the metal or metals of interest(e.g., iron, copper, zinc, nickel, etc.), and that is compatible withother components to which it comes into contact, may be used.

Referring now to FIG. 3, in one particular embodiment, wherein the sawkerf or exhausted slurry comprises an organic lubricating fluid, suchmineral oil, a chelating agent that is soluble in the organiclubricating agent is added thereto (the saw kerf optionally beingallowed to age for an appropriate period of time, either before or afterhaving added the chelating agent thereto, to allow time for metals, suchas copper and/or nickel, to diffuse from the bulk of the siliconparticles present therein). A suitable chelating agent may be selectedfrom among those generally known in the art, including for example knownaldoxime chelating agents (e.g., 5-nonyl-2-hydroxybenzaldoxime, or“P50”), which are known to be effective to sequester, for example,copper. The resulting slurry may then be contacted and agitated (e.g.,shaken) with an aqueous, acidic solution (e.g., a solution have a pHbetween about 1 and less than about 7, or between about 2 and about 3,such as a dilute HCl solution). The silicon particles, which arehydrophilic, will transfer to the aqueous phase of the resultingmixture, leaving the bulk of the chelate-metal complex in the organicphase. After allowing sufficient time for the two phases to separate,the aqueous phase may be collected or separated using conventionalmethods generally known in the art.

Regardless of the means by which the saw kerf is initially treated forpurpose removing metal particles and/or oxide particles therefrom(including from the bulk of the silicon particles themselves), theresulting portion thereof that includes the silicon particles (e.g., thesolid particle mixture that remains after treatment with the aqueousacidic solutions, or the aqueous phase resulting from the liquidextraction used when the lubrication fluid is organic), may be furthertreated as necessary (e.g., filtered to remove excess water, or dried,or used directly), in order to suitably prepare the solids for asubsequent solid separation to isolate and recover the desired siliconparticles (the solid separation being further detailed herein below),and optionally recover the abrasive grains as well.

C. Froth Flotation

In another embodiment, the saw kerf may be contacted (e.g., rinsed orwashed) with a mixed aqueous solution of hydrochloric acid (HCl) andhydrofluoric acid (HF), in order to dissolve and remove metal particlesor contaminants (HCl solution), and oxide particles or contaminants (HFsolution) present therein, and/or to remove metal contaminants presenton or in (e.g., present in the bulk of) the silicon particles, andfurther to effectively separate the silicon particles from thecontaminants dissolved in the solution. Specifically, it is to be notedthat, in this solution, silicon is hydrogen terminated, and therefore ishydrophobic. It is therefore attracted to gas bubbles that form on itssurface, while in the presence of HF. A froth is generated as a resultof the volatile fluorides that are formed, and floating at or near thetop of this froth is the visible gray sheen of silicon particles.

The concentration of the respective acids in solution, and/or the ratiosof the two acids in solution to each other and/or the concentration ofsilicon present in the saw kerf, may be optimized experimentally usingmeans generally known in the art, in order to maximize siliconseparation from the saw kerf, and/or to maximize dissolution (and thusremoval) of metal and/or oxide contaminants present therein.Additionally, the concentrations and/or ratios of the two acids may bealtered to ensure an appropriate or sufficient amount of froth is formed(e.g., a sufficient amount to effectively suspend or float the siliconpresent therein). Typically, however, a ratio of about 75 to about 125ml, or about 90 to about 110 ml, and in particular about 100 ml of adilute HCl solution (0.5 to about 0.25 normal (N), or about 0.75 toabout 0.2 N, and in particular about 0.1 N), is mixed with between about5 and about 20 ml, or about 8 and about 16 ml, of an HF solution (the HFconcentration therein being between about 40% to about 60% (by weight),or about 45% to about 55%, and in particular about 49%). This solutionis then typically applied to or mixed with about 20 to about 50 grams,or about 25 to about 40 grams, and in particular about 30 grams, ofsolids (e.g., saw kerf material), resulting in a froth that is about 5to about 8 time the volume of the initial liquid+solid mixture.

As disclosed in the working Examples below, a exemplary hydrochloricacid and water mixture or solution may have a ratio of about 1:9 36%HCl:H₂O, while the frothing solution may comprise a mixture of H₂O, HF(49% solution) and HCl (a 36% solution) in a ratio of about 10:3.125:1(e.g., about 250 ml:80 ml:25 ml).

Once formed, the froth may be separated from the remaining liquid orsolution, using means generally known in the art (e.g., filtration orskimming). Once collected, the froth may be dried for later solidseparation, or directly subjected to an appropriate solid separationtechnique (e.g., density solid separation), as further detailedelsewhere herein.

3. Solid Separation

After the saw kerf have been treated to remove metal and/or oxideparticles and contaminants, the resulting solid particulate mixture,which comprises silicon particles and abrasive grains (e.g., siliconcarbide particles), may be separated using one or more means generallyknown in the art, including for example means capable of separatingparticles based on mass (e.g., weight or density) or size. Exemplarydevices suitable for this type of separation include a hydro-cycloneseparator or a sedimentation centrifuge, both devices being commerciallyavailable. Alternatively, and as further detailed herein below, providedthere is an adequate difference in the diamagnetic properties of thesolids that are to be separated (such as, for example, the differencesbetween silicon and silicon carbide particles), a magnetic fieldgradient may be applied to the particles to separate them.

In this regard it is to be noted while the solids may be separated aftertreatment to remove metal and/or oxide particles or contaminants, in analternative approach the solid particles may first be subjected to someform of separation technique (as noted above), provided the contaminantsdo not prevent an acceptable amount of the silicon particles from beingseparated from the abrasive grains. For example, in some instances thesilicon particles may be bound in some way to the abrasive grit, and inparticular to silicon carbide present. If these bonds can be broken,centrifugation may be an acceptable method for separating the variouscomponents of the saw kerf, because the silicon particles, the abrasivegrit (e.g., silicon carbide), the metal and the oxide particles havesufficiently different weights or densities to allow for separation(metals and metal oxides typically having substantially higher densitiesthan silicon and silicon carbide). The resulting silicon particles maythen be subjected (or optionally subjected) to one or more of thetechniques previously detailed above, in order to remove the metaland/or oxide particles or contaminants still present therein.

A. Mass Separation

Typically, the materials that are to be separated by, for example,centrifugation in the solid particulate mixture are silicon, which hasan average density of about 2.33 g/cm³, and silicon carbide, which hasan average density of about 3.22 g/cm³. Additionally, silicon dioxidehas an average density of about 2.26 g/cm³, but this can generally beremoved by an acid wash or treatment of some kind as previously detailedherein, silicon dioxide for example being dissolved in a hydrofluoricacid solution.

Centrifugal separation of the solids may optionally being aided by theaddition or use of a “heavy liquid,” the appropriate liquid beingselected to have a density sufficiently close to the materials beingseparated to aid in the separation. For example, when separating siliconand silicon carbide particles in this way, the density of the heavyliquid or fluid used in the separation will typically be between about 2g/cm³ and about 3.5 g/cm³, and preferably will be between about 2.3g/cm³ and about 3.2 g/cm³.

In this regard it is to be noted that there are several potentiallysuitable fluids that can be used in the heavy liquid centrifugalseparation, the term “heavy liquid” being generally known in the art.Examples of suitable heavy liquids, and their respective densities,include: iodomethane (CH₃I), 2.2789 g/cm³ at 20° C.; diiodomethane(CH₂I₂), 3.325 g/cm³ at 20° C.; bromomethane (CHBr₃), 2.889 g/cm³ at 15°C.; tetrabromomethane (CBr₄), 2.961 g/cm³ at 100° C. and 3.420 g/cm³ at20° C.; hydrogen iodide (HI), 2.850 g/cm³ at −47° C. and 2.797 g/cm³ at−35.36° C.; bromine (Br₂), 3.1028 g/cm³ at 20° C.; potassium fluoridemixture (KF.2H₂O), 2.420 g/cm³ at 20° C.; and, aqueousheteropolytungstates, which typically have densities from about 1 to 3g/cm³ at 20° C.

The suitable heavy liquids have differing properties and costsassociated with them. For example, iodomethane and diiodomethane aregood for microscale tests when mixed with each other, but can be toxicand mutagenous. Bromomethane may be ideal for microscale testing, butcan also be toxic and mutagenous. Hydrogen Iodide can be toxic as well,but does provide an ideal density separation for silicon and siliconcarbide. Bromine can be toxic and difficult to handle due to its highvapor pressure. Further, bromine has been known to cause health effectsassociated with mental capacity. Accordingly, selection of a suitableheavy liquid will keep these issues in mind and will be accompanied byappropriate safety measures.

The potassium fluoride mixture (KF.2H₂O) has a number of advantages. Forexample, it has a relatively low cost, the centrifugal separation can bedone in the liquid state, and the silicon layer can be scraped off afterfreezing of the container (and contents thereof). The mixture can alsopotentially etch silicon oxide on silicon, which can be advantageousduring separation operations to break up chemically bonded agglomeratesof silicon and silicon carbide, when bonded by thin layers of nativesilicon oxide. (See, e.g., H. Noguchi and S. Adachi, Chemical treatmenteffects of silicon surfaces in aqueous KF solution, Appl. Surf. Sci.,Vol. 246, Issues 1-3, pages 139-48 (Jun. 15, 2005)).

Of the suitable heavy liquids, hydrogen iodide, bromine, bromomethane,the potassium fluoride mixture and the aqueous polytungstates aretypically the most practical candidates for use. Suitable aqueouspolytungstates include polyoxytungsten alkali salts that are capable ofbeing partly substituted with any of the group consisting of potassium,arsenic, silicon, germanium, titanium, cobalt, iron, aluminum, chromium,gallium, tellurium, boron, iodine, nickel, molybdenum, beryllium,platinum, and the like. Suitably, sodium polytungstate (SPT) and lithiumpolytungstate (LPT) are used as the heavy liquids.

In an alternative embodiment to the use of a heavy liquid, or inaddition to the use of a heavy liquid, the silicon may optionally beconverted to a silane, and more specifically SiI₄, by means generallyknown in the art. While the silanes could be distilled and re-depositedor collected in the form of clean [poly]silicon, it may be preferable,due for example to the cost and/or complexity of such a process, to usethe silane (e.g., SiI₄ form in the centrifugal separation process,particularly given that cryogenic hydrogen iodide, which may be used orformed therein, has a density within the range bounded by the silaneparticles themselves and the abrasive grains (e.g., silicon carbideparticles).

It is to be noted that conversion to SI₄ is a potentially attractiveoption, particularly due to the potential for carrying out theconversion in a cost-effective manner. Iodine may be anaturally-occurring element of a silicon/silicon carbide separationprocess, and an iodide-based reactor process is known in the art to bereadily available (See, e.g., Cisek, et al., Solar Grade Silicon fromMetallurgical Grade Silicon Via Iodine Chemical Vapor TransportPurification, National Renewable Energy Laboratory Publication,NREL/CP-520-31443 (May 2002)).

It is to be noted that the process conditions used for solid separation(e.g., type of equipment used, number of cycles or revolutions throughthe device, or duration of the separation process), and/or the typeand/or quantity of the separation aid (e.g., heavy liquid), used may bedetermined experimentally using techniques generally known in the art(and/or as further illustrated in the working Examples provided below).Additionally, or alternatively, it is to be noted that the resultingcontents of the centrifuge container may optionally be frozen, in orderto aid with recovery of the silicon particles.

B. Separation by Magnetic Field Gradient

As previously noted, silicon and, for example, silicon carbide, havedifferent diamagnetic properties that can be used to separate them by astrong magnetic field gradient, which may be configured or designedusing techniques and methods generally known in the art. Conventionally,there are three ways to make a magnetic separation. The first methodinvolves water, the second method involves air, and the third methodinvolves oxygen (which is a paramagnetic material). During the oxygenseparation method, the magnetic forces on diamagnetic materials can beenhanced. (See, e.g., patents referring to paramagnetic fluids aiding inlevitation of diamagnetic materials in magnetic fields, EP1181982, U.S.Pat. Nos. 7,008,572 and 6,902,065). Applying these known magneticseparation techniques, in combination with the use of permanent magnets,may present a cost-effective manner in which to magnetically separatethe particles of interest in the present disclosure. The permanentmagnet may be, for example, a neodymium magnet of the NdFeB type, and inparticular the permanent magnet may be Nd₂Fe₁₄B.

As is generally recognized in the art, diamagnetic materials arerepelled from magnetic fields by a force proportional to HgradH.Accordingly, and referring now to FIG. 5, a magnet arrangement used formaximum HgradH at the center is illustrated, which is suitable for usein accordance with the present disclosure for separation of siliconparticles from other particles present in the mixed solid particulatesample (e.g., silicon carbide particles). This arrangement produces anon-uniform magnetic field of about 3 Tesla between a first and a secondpole. The magnetic field lines in FIG. 5 display the cross section of atoroid around an air core. With the design of FIG. 5, it is possible torun a capillary tube down the center with particles flowing inside. Acombination of gravitational forces and magnetic forces will then forceparticles to either side of the capillary.

Referring now to FIG. 6, it is possible to calculate HgradH in the coreof the magnetic field from left to right, which can then be used tocalculate the forces on the particles within the field. The presentdisclosure found that in small areas, it is possible to obtain an HgradHfrom about 5·10¹⁵ A²/m³ to about 6·10¹⁵ A²/m³. The terminal velocity ofa sphere in a fluid is calculated by:

$\begin{matrix}{{v(r)} = {\frac{{- 2}r^{2}}{9\eta_{fluid}}\left( {{g\left( {\rho_{Solid} - \rho_{fluid}} \right)} + {\mu_{0}{\rho_{Solid}\left( \chi_{{Solid}{({mass})}} \right)}}} \right){H\left( \frac{H}{z} \right)}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where:

-   -   χ_(water(mass))=−7.194·10⁻⁷ cm³/g at 293K;    -   χ_(Si(mass))=−1.112·10⁻⁷ cm³/g;    -   g is acceleration of gravity in the z-direction;    -   μ_(o)=4π·10⁻⁷ newton/ampere²;    -   v(r) is the terminal velocity of the particle or radius r in the        z-direction in water of viscosity η;    -   ρ is the density of the material denoted by the subscript of ρ        (thus, ρ_(fluid) is the density of the separating fluid, e.g. in        kg/m³);    -   z is the direction of force exerted by gravity (i.e., it is the        coordinate of “up” and “down”); and,    -   H is the magnetic field in units of Amperes/meter (A/m).

Referring now to FIGS. 7 and 8, by using the data displayed therein, itis possible to calculate for expected particle size ranges the terminalvelocity of particles in water. Due to the combination of forces ofmagnetism, liquid viscous drag, gravity, and broad size distribution,separation by this magnetophoresis can be difficult in water. If,however, the particles are dried and suspended separated in air (asopposed to bonded to each other), and HgradH is increased to 2·10¹⁶A²/m³, then it is possible to produce separating motion based on theparticular chemical identity of the particle. Through the use of liquidoxygen or liquid air, the permanent magnets can support the particles.

Referring again to FIG. 6, and now to FIG. 9, the central region of FIG.9 illustrating the center of FIG. 6 in schematic form, it is to be notedthat by utilizing the magneto-Archimedes effect, the materials can beplaced into the configurations of FIGS. 5 and 6 and rotated so thatthere is an upper and lower chamber or regions, as illustrated in FIG.9. By pumping an aerosol of dry particles in liquid air into the upperchamber, only the silicon particles would fall through to the lowerchamber under the force of gravity. Meanwhile, silicon carbide andsilicon dioxide would remain magnetically suspended in the upperchamber. This configuration is also known as a magnetic filter. As shownin FIG. 10, wherein the terminal velocities are mm/min, essentially onlysilicon can be forced through the gap under gravity, whereas siliconcarbide and silicon dioxide are suspended in the upper chamber.

It is to be noted that it may be possible to apply extra force on theparticles by using a paramagnetic material (e.g., oxygen), whereχ_(fluid(mass)) has a large and opposite sign to χ_(Solid(mass)) inEquation 1.

FIG. 11 further displays a practical implementation of the disclosedmethod. FIG. 11 is a magnetic arrangement of FIG. 6 extended as aninclined tube structure several meters long. The aerosol is injected atone end and at the other end the lower channel feeds a silicon bin andthe upper channel feeds a kerf waste bin. That is, the silicon carbideand silicon dioxide are fed into a bin that collects particles from theupper chamber, whereas the silicon particles are fed into a separate binthat collects particles from the lower chamber, as depicted in FIG. 11.In the implementation of FIG. 11, liquid air may be needed in someinstances, such as for 40 Moe permanent magnets. Liquid air has aviscosity of 0.173 cP at −192.3° C., a density of 0.87 g/cm³, and aparamagnetic susceptibility of approximately 146·10⁻⁶ cm³/g. In liquidair, the value of HgradH that may be needed to form a magnetic filtercan be greatly reduced. The exact field strength×gradient can be anarrow range for a liquid air mixture, and determined experimentally.

It may also be possible to separate the silicon carbide particles intothe lower chamber. Referring now to FIG. 12, it can be seen that siliconcarbide will fall through the magnetic filter to the lower chamber whenHgradH=0.037·10¹⁶ A²/m³.

In this embodiment, liquid air, the strength of the magnets that may beneeded for a separation is reduced by a factor of 14, which can bringthe magnetic field requirements within reach of existing permanentmagnet technology. Thus, although reducing the oxygen content of themixture might require stronger magnets, it could then be possible togain a wider working range of magnetic field intensity and gradients. Asa result, instead of having difficulty arranging a magnetic field with adifficult specification, the liquid air composition can be adjusted fora specific set of magnets. For liquid air, the working range for maximumHgradH is from about 0.034·10¹⁶ A²/m³ to about 0.037·10¹⁶ A²/m³. If theliquid air composition is 10.5% O₂ and 89.5% N₂, the working range isfrom about 0.066·10¹⁶ A²/m³ to about 0.074·10¹⁶ A²/m³. The lattercomposition would have the same magneto-Archimedes effect if thecompressed air has an oxygen partial pressure of about 6.9 MPa (1000psi).

It is to be noted that an HgradH of 0.25·10¹⁶ A²/m³ can be achieved withpermanent magnets, allowing separation with oxygen partial pressure ofabout 1.8 MPa (265 psi).

It is to be further noted that the design of the magnetic field/systemdetailed herein may be optimized as needed by means generally known inthe art, and detailed herein, for a given saw kerf material (e.g., if adifferent abrasive grain is present), without departing from the scopeof the present disclosure. In one embodiment, however, the tubularmagnetic filter suitably has an hour-glass cross-section and thenon-uniform magnetic field has a magnetic flux density of about 3 Teslabetween a first pole and a second pole. Further, the silicon kerfparticles in the magnetic field gradient can be separated by pressurizedgas, liquids, or cryogenic fluids. More suitably, the silicon kerfparticles are suspended in pressurized gas.

4. Additional Applications

It is to be noted that, in addition to the embodiments detailed hereinabove, the present disclosure provides a means by which to recover theabrasive grains from the saw kerf. Once collected, “spent” abrasivegrains may be separated from “unspent” abrasive grains by meansgenerally known in the art (see, e.g., U.S. Pat. No. 7,223,344, theentire contents of which are incorporated herein by reference for allrelevant and consistent purposes), the latter being reused in furtherslurries.

Additionally, the present disclosure provides a means by which toprepare a new silicon raw material, such as solar grade silicon pellets.Specifically, the silicon particles recovered from saw kerf, using thevarious embodiments disclosed in the present disclosure, may be furtherprocessed (e.g., melted and shaped) to form solar grade silicon pellets,using means generally known in the art.

Finally, the present disclosure provides an improved process for slicingor cutting silicon ingots, which involve the addition of a chelatingagent to the slurry used in the process. The presence of the chelatingagent, such as a chelating agent soluble in an organic lubricating fluid(e.g., P50 in combination with a mineral oil lubricating fluid) enablemetal contaminants (such as those introduced from the wire saw) in thesaw kerf to be trapped or sequestered. In this way, subsequent stepstaken to recover the silicon particles, and/or abrasive grains, and/orthe lubricating fluid, form the saw kerf may be simplified.

The following Examples describe various embodiments of the presentdisclosure. Other embodiments within the scope of the appended claimswill be apparent to a skilled artisan considering the specification orpractice of the invention as described herein. It is intended that thespecification, together with the Examples, be considered exemplary only,with the scope and spirit of the invention being indicated by theclaims, which follow the Examples.

EXAMPLES

The following non-limiting examples are provided to further illustratethe present disclosure.

Example 1 Exemplary Si Particle Separation

Approximately 3 grams of silicon were prepared from 190 grams ofpolyethylene glycol (PEG) suspended saw slurry using SPT as the workingheavy liquid. The PEG was removed by repeated washing with water andvacuum filtration. When removed, the material of bulk liquid resembles ahard mud, which could be somewhat re-suspend on mixing with water.

Metals were removed from the mixture by repeated washing with 0.1 normalHCl and vacuum filtration. HCl was used at this step without HF, so thatcopper may be removed. This is so because in the presence of HF, copperand nickel may tend to plate out on the silicon surface.

The next step involved treating the washed solids with HF:HCl, whichre-suspended the particles and generated copious amounts of gas bubblesand formed a floatation froth. A grey slick of silicon was observed onthe top surface of the froth, while the liquid underneath the froth wasbrown and comprised some portion of the silicon carbide.

In this regard it is to be noted that, without being held to aparticular theory, it is believed that metal from the wire saw acted tocreate bonds between the silicon and carbide present in the solidmixture. The treatment of the solid mixture with the acids acts tode-bond the silicon from the silicon carbide. After de-bonding, thematerial was washed with water and vacuum filtered until the pH of thefluid is approximately 7.

The resulting wet mud-like mixture was then re-suspended in a heavyliquid (in this instance SPT) and centrifuged at approximately 17,000 gfor approximate 70 minutes. During centrifugation, the top of the vialbecame enriched in silicon. In order to prevent the collisions of heavyparticles with light particles and forcing silicon downwards, the solidsloading in the vial was limited to about 5% by volume.

The collected silicon enriched material was then captured andcentrifuged a second time with a clean mechanical separation. Thesilicon was then collected in a centrifuge-filter and the suspending SPTwas largely removed.

In this instance, the SPT was diluted with water to a density of about2.6 g/cm³ and a viscosity of about 10 cP. This represents the midpointdensity between silicon and silicon carbide and was believed to be anideal density to analytically separate same-sized particles. The sizedistributions of the particles were then taken and essentially noparticles smaller than 100 nm was found to present. Particle sizemeasurements showed that about 99% of the silicon particles were largerthan about 500 nm; as such, the optimum density and viscosity of the SPTliquid could be adjusted for separating 100 nm silicon carbide particlesfrom 500 nm silicon particles.

FIG. 13 illustrates and optimum density for SPT, determined by thediffering size cut-offs used for silicon and silicon carbide particlesin this experiment, and the minimum density that may be needed forseparation (the optimum density is circled). FIG. 14 further discloses apractical range of densities for the SPT and the minimum separationdensity of SPT compared to the symmetric sedimentation velocities of thesilicon and silicon carbide particles.

In another test, polysilicon dust was rinsed with about 0.1 ml to about0.3 ml of water. The material was then centrifuged five times at about4,000 g each time, which forces water through the packed solid, and afilter membrane that was present therein. (In this regard it is to benoted that although 5 centrifuge passes were calculated here, fewercentrifuge passes may be used within a purpose-built, automated andoptimized system.)

An HCl solution (approximately 0.1 N) was then used to rinse thecentrifuged material (about 0.1 ml to about 0.3 ml), and then once againit was centrifuged five times at about 4,000 g each time. The vial wasthen filled with the HCl solution again, so that all of the silicon dustwas covered. The resulting mixture was then aged for 24 hours (to helpremove bulk metal contaminants, as further detailed in the followingExample).

After aging, the silicon kerf waste material (now polysilicon dust) wasrinsed with about 0.1 ml to about 0.3 ml of water and then centrifugedfive times at 4,000 g each time once again. The resulting silicon solidswere then dried for about 12 hours at about 40° C.

Example 2 Bulk Metal Contamination

As previously noted, some metal contaminants may be out-diffused fromsilicon particles within a commercially reasonable period of time. Byusing the following diffusion equation series solution, it is possibleto calculate the bulk metal out-diffusion from the particles. The modelis a uniformly bulk contaminated sphere, where the copper/nickel isforced to zero on the boundary.

$\begin{matrix}{{u\left( {r,t} \right)} = {u_{0}\frac{2a}{\pi \; r}{\sum\limits_{n = 1}^{\infty}\left\lbrack {\frac{\left( {- 1} \right)^{n + 1}}{n}{\sin \left( \frac{n\; \pi \; r}{a} \right)}{\exp\left( \frac{{- n^{2}}\pi^{2}{Dt}}{a^{2}} \right)}} \right\rbrack}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

Where u is the transient and u_(o) is the initial concentration ofcontaminant, D is the diffusion coefficient, a is the particle radius, ris the radius of the particle in question, n is an integer in thesummation series and t is time. The function u(r,t) is well-behaved, andthe total amount of material in the spherical particle at any time canbe determined by a term-by-term integral on r.

$\begin{matrix}{{I(t)} = {\int_{0}^{a}{{u\left( {r,t} \right)}4\pi \; r^{2}{r}}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

Equation 3 represents an integration with respect to the radius of themetal content of the sphere at any given time, t. Thus, if Equation 2were plugged into Equation 3 for u (r,t), the normal procedures ofcalculus lead to Equation 4:

$\begin{matrix}{{I(t)} = {u_{0}\frac{8a^{3}}{\pi^{2}}{\sum\limits_{n = 1}^{\infty}\left\lbrack {\frac{\left( {- 1} \right)^{n}}{n^{3}}\left( {{- {\sin \left( {n\; \pi} \right)}} + \left( {n\; \pi} \right)} \right){\exp\left( \frac{{- n^{2}}\pi^{2}{Dt}}{a^{2}} \right)}} \right\rbrack}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

In these equations, when a=1 and D=1, the time dependant out-diffusionof material yields the results disclosed in FIGS. 15A and 15B.

In the first sample that was separated, the bulk content of copper was47 ppma and the bulk content of nickel was 0.21 ppma. It can be notedthat if the tests were run on Czochralski silicon ingots, as opposed tosolar grade ingots, the bulk copper may be reduced by a factor of about47,000 and the bulk nickel may be reduced by a factor of about 105.

For copper in 10 ohm-cm silicon at 20° C., the diffusion coefficient is364 μm²/min. For P(++) silicon the copper diffusion coefficient is 0.371μm²/min. In P(−) silicon at 20° C., the nickel diffusion coefficient is0.427 μm²/min. The following Table 3 discloses the calculations ofout-diffusion time of bulk metals for a 7-micron, oxide terminatedsilicon particle, at 20° C.

TABLE 3 Calculated out-diffusion time of bulk metals for a 7-micron,oxide terminated silicon particle, at 20° C. Initial bulk Final bulktime to contamination contamination final Material metal (ppma) (ppma)(minutes) Si 10 ohm cm Cu 47 0.001 0.14 Si 10 ohm cm Ni 0.21 0.002 0.48Si about 0.005 ohm Cu 47 0.001 137 cm

It can be noted that by waiting a few hours at room temperature, bulknickel and bulk copper will plate out onto the surface of an oxidecoated particle.

Example 3 Froth Flotation

A procedure for initial separation and enrichment of silicon particlesusing froth flotation was performed by hand to substantially lower themass-load needed for a centrifugal based separation system. In thisExample, 47.0818 grams of saw kerf particles were treated to producesilicon enriched froth (the saw kerf being from a different batch thanthose analyzed and shown in Table 1). A vacuum filter of 250 ml liquidcapacity was used with a 0.45 micron PVDF filter for rising andde-watering.

The following sequence of steps were used here:

-   -   (1) the waste was rinsed with 250 ml H₂O twice;    -   (2) rinsed with 1,000 ml solution of 1:9 36% HCl:H₂O;    -   (3) drained to a sludge and recovery manually to a PTFE beaker;    -   (4) to the resulting sludge added a frothing solution: H₂O:49%        HF:36% HCl 250 ml:80 ml:25 ml;    -   (5) the froth was generated and skimmed away to another beaker        over a period of 7 hours;    -   (6) the captured froth was rinsed in vacuum filter using 1,800        ml:200 ml H₂O:36% HCl;    -   (7) captured froth rinsed in the vacuum filter with 1,000 ml        H₂O;    -   (8) the recovered paste was dried and weighed;    -   (9) the paste was etched to remove silicon using 4:3:1 H₂O:49%        HF:70% HNO₃ until gas generation stopped; and,    -   (10) the surviving material was dried and weighed.

The recovered dried froth contained 0.3085 grams silicon carbide and0.3295 grams silicon. The recovered silicon was expected to be about0.011*47.0818 (see Table 1) or about 0.52 grams. As the froth in thissample contains more than expected silicon, the gross separation ofsilicon from silicon carbide was achieved with 0.3295/0.52*100 equalsabout 63% efficiency, and an enrichment factor on the order of(0.3295/(0.3295+0.3085))/0.02 equals about 26.

Example 4 SEM Imagery

For SEM imagery and size distributions, samples of silicon particleswere collected on a 0.45 micron polyvinylidene difluoride (PVDF)membrane filter and were then acid washed. The resulting particles wereirregular in shape and gave the appearance of material that was cut bybrittle fracture (See FIGS. 16A and 16B).

FIGS. 16A and 16B display images of silicon kerf particles separatedfrom the saw kerf mixture. A total of 125 silicon particles weremeasured for cross-sectional area. The images were analyzed by tracingover the individually distinguishable particles computing thecross-sectional area, and the equivalent sphere diameter of the samecross-sectional area. The resulting size distribution compares to thedata collected in Examples 1 and 2 by a PSS780 Accusizer particle sizeanalyzer.

Example 5 Bulk Metal Digestion (First Attempt)

When using SEM analysis, material captured on the PVDF membrane filterwas found to be free of tungsten. Due to the possible presence offluorine, however, any iron signal that might be present could bemasked. As a result, EDX spectra of four different silicon particleswere taken to determine the amount of bulk metal contamination, if any,within the silicon particles.

FIGS. 17A-17D depict the results of the EDX spectra for the fourdifferent silicon particles examined. The iron and fluorine signals werefound to be difficult to differentiate. The fluorine and carbon signalsin FIGS. 17A-17D come from the PVDF filter used to capture the siliconparticles.

Initial analysis of the results showed particles that were notcompletely free of metals. Further testing revealed, however, that thepurity of the silicon kerf particles can be improved by increasedleaching and rinsing. Table 4 displays the bulk metal digestion data ona 14 mg silicon particle from the first separation attempt. Because ofthe volatility of boron and phosphorous in the technique used, the boronand phosphorous measurements may not be relevant.

TABLE 4 Bulk metal digestion data on 14 mg silicon Element ppma Elementppma Element ppma Element ppma B 11.5 K 9.4 Fe 1.4 Sr 0.035 Na 3.8 Ti2.1 Ni 0.21 Mo 0.023 Mg 7.8 V 1.0 Co 0.0027 Ba 0.010 Al 2.0 Cr 0.087 Cu46.7 Ta 0.00044 P 3.0 Mn 0.037 Zn 0.16 W 16.3

Example 6 Polysilicon Pellets

In order to prepare polysilicon pellets, purified silicon was separatedfrom the kerf particles and dropped into a silica crucible and melteddown into a pellet. The dried polysilicon dust was able to bind togetherto form a weak pellet. After drying, however, due to shrinkage thepolysilicon dust is bound weakly to the polypropylene wall of thecentrifuge filter. The centrifuge filter vials have a 100 nm PVDF filtermembrane and an outer vial diameter of 8 mm.

The pellet was then cooled from the melt at a rate of no slower thanabout 5.7K/min, and then released from the crucible by etching off thesilica with 49% HF and 36% HCl at a ratio of 1,000:25 to prevent thedeposition of copper. The resulting surface was rough with texture about3 microns long. The interior of the surface is homogeneous under opticalmicroscopy.

About 20 mg of flakes that broke from the pellet (while being removedfrom the crucible) were exposed to a 1:1 mixture of 49% HF and 70% HNO₃.The silicon dissolved completely and no residue was observed. Such aresult is an indication that silicon carbide contamination may not havebeen present in gross quantities.

After the pellet was removed from the crucible, the etching was quenchedwith a 1,000:25 solution of de-ionized water and 36% HCl, and thensoaked in 1,000:25 solutions of 30% H₂O₂ and 36% HCl in order to grow aclean protective oxide on the surface.

The resulting polycrystal was full of voids and had a columnar-typeinternal structure. There was enough internal mechanical stress in thepolycrstyal that when handled it broke. FIGS. 18A-18D depict Nomarksiimicroscope images of flakes broken off of a silicon pellet, showingexterior and interior surfaces.

Example 7 Bulk Metal Digestion (Second Attempt)

In this Example, 164 mg of the polycrystal were digested in HF:HNO₃. Thebulk metal content was improved from the first attempt as a result ofadditional acid washing of the recovered silicon powder (compare Table 4and Table 5).

TABLE 5 Bulk metal digestion data on 14 mg silicon from secondseparation attempt, after melting element ppma element ppma element ppmaelement ppma B 3.730 K 0.001 Fe 0.357 Sr 0.000 Na 0.001 Ti 0.459 Ni0.039 Mo 0.013 Mg 0.036 V 0.094 Co 0.001 Ba 0.000 Al 0.128 Cr 0.051 Cu2.638 Ta 0.000 P 0.507 Mn 0.006 Zn 0.001 W 8.279

Metal segregation effects from melting may not be significant as thelocation of the polysilicon used for acid digestion was not selective.The digestion fluid was centrifuged at 21,000 g for 2 hours and theresults are detailed in Table 5. The dry content produced was greensilicon carbide, measured at 8.41 mg (a weight fraction of 0.0513). Thestarting weight fraction of silicon carbide is 0.979, before the twocentrifuge passes performed. Each centrifuge pass can be taken as afilter, and a fraction C_(f) carbide survives on each pass. Thedetermination of the purification coefficient C_(f) per centrifuge passcan be found through algebraic equations.

Prior to purification, the total weight of solids is W⁰, the weight ofthe silicon is W_(Si) ⁰, and the carbide is W_(SiC) ⁰. The ratio ofsolids to each from this sample based on previously disclosedcalculations is W_(Si) ⁰/W_(SiC) ⁰=0.0216. The mole fraction of carbon,and mole fraction of silicon carbide fa⁰, are the same. Prior topurification, therefore, the following conditions are present:

$\begin{matrix}{{W^{0} = {W_{Si}^{0} + W_{SiC}^{0}}}{{fa}^{0} = \frac{\frac{W_{SiC}^{0}}{{MW}_{SiC}}}{\frac{W_{Si}^{0}}{{MW}_{Si}} + \frac{W_{SiC}^{0}}{{MW}_{SiC}}}}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

It should be noted that the superscripts in this notation representindices, not powers.

After each centrifuge step, approximately all of the silicon isretained, but only a small fraction C_(f) of carbide remains. After ncentrifuge steps the following relationship is present:

$\begin{matrix}{{W^{n} = {{W_{Si}^{0} + {\left( C_{f} \right)^{n}W_{SiC}^{0}}} = {W_{Si}^{0} + W_{SiC}^{n}}}}{{fa}^{n} = {\frac{\frac{W_{SiC}^{n}}{{MW}_{SiC}}}{\frac{W_{Si}^{0}}{{MW}_{Si}} + \frac{W_{SiC}^{n}}{{MW}_{SiC}}} = \frac{\frac{\left( C_{f} \right)^{n}W_{SiC}^{0}}{{MW}_{SiC}}}{\frac{W_{Si}^{0}}{{MW}_{Si}} + \frac{\left( C_{f} \right)^{n}W_{SiC}^{0}}{{MW}_{SiC}}}}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

where W^(n) is the total mass after n centrifuge passes, n in theexpression (C_(f))^(n) is a power and not an index, and fa^(n) is themole fraction of carbide in the mixture.

After two centrifuge passes, 0.163 g of silicon and 0.00841 g of carbideremained and were taken for analysis. Thus, the starting amount ofsolids was 0.163/0.0216=7.593 grams of solids. Based on startingmaterial proportions, 7.593*0.0216=0.663 grams of silicon, and(1−0.0216)*7.593=7.429 grams of carbide.

Thus, it follows that:

$\begin{matrix}{{C_{f} = \left( \frac{W_{SiC}^{n}}{W_{SiC}^{0}} \right)^{1/n}},{{{or}\mspace{14mu} \left( \frac{0.00841}{7.429} \right)^{1/2}} = {0.0336.}}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

It remains possible, however, to use the relationships previouslydeveloped to determine the number of centrifuge passes needed to reducethe carbon content to SEMI standard levels for polysilicon. SEMIstandard M6-1000 states required carbon content as <=10 ppma for singlecrystal solar cells, and <=20 ppma for polycrystalline cells. Using thesample of saw kerf used in experimentation, the following relationshipexists for calculating the number of n centrifuge passes that may beneeded to remove carbon (as carbide) below specification limits.

$\begin{matrix}{{{ppma}\mspace{14mu} {Carbon}} = {{10^{6}{fa}^{n}} = {10 = {\frac{\frac{\left( C_{f} \right)^{n}\left( {1 - 0.0216} \right)}{{MW}_{SiC}}}{\frac{0.0216}{{MW}_{Si}} + \frac{\left( C_{f} \right)^{n}\left( {1 - 0.0216} \right)}{{MW}_{SiC}}}10^{6}}}}} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$

It takes n=4.41 to arrive at the SEMI spec level (10 ppma) for singlecrystal cells. As such, at least 5 centrifuge passes may be needed toget carbide down to acceptable levels.

SPT liquid was used for this proof of concept. SPT has the advantage oflow toxicity, and liquidity at room temperature. Gross metalliccontamination is a problem, however, particularly with tungsten. Thesalt-hydrate KF.2H₂O has the right density for the present disclosure, afurther advantage of attacking particles bonded with SiO₂, and a lowercost. Despite the thermal requirements, KF.2H₂O is suitably thepreferred fluid for centrifugal separations. High temperature may beneeded, however, to get useable viscosity.

In view of the above, it will be seen that the several advantages of thedisclosure are achieved and other advantageous results attained. Asvarious changes could be made in the above processes and compositeswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

When introducing elements of the present disclosure or the variousversions, embodiment(s) or aspects thereof, the articles “a”, “an”,“the” and “said” are intended to mean that there are one or more of theelements. The terms “comprising”, “including” and “having” are intendedto be inclusive and mean that there may be additional elements otherthan the listed elements. The use of terms indicating a particularorientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience ofdescription and does not require any particular orientation of the itemdescribed.

1. A method for slicing a silicon ingot, the method comprisingcontacting a surface of the silicon ingot with a reciprocating wire sawand a slurry comprising an organic lubricating fluid, an abrasiveparticulate, and a metal chelating agent soluble in the organiclubricating fluid.
 2. The method of claim 33, wherein the organiclubricating fluid is mineral oil.
 3. The method of claim 34, wherein thechelating agent is an aldoxime.
 4. The method of claim 1 wherein theabrasive particulate is silicon carbide.
 5. The method of claim 1wherein contacting the surface of the silicon ingot with thereciprocating wire saw cause silicon particles from the ingot to beremoved and form part of the slurry, the method further comprising:mixing the slurry with an aqueous acid solution and allowing the mixtureto separate into an aqueous phase and an organic phase, the aqueousphase comprising silicon particles and the organic phase comprising acomplex formed between the chelating agent and metals; collecting theaqueous phase comprising the silicon particles; and recovering at leasta portion of the silicon particles from the aqueous phase.
 6. The methodof claim 5 wherein the recovered silicon particles have a carbon contentof less than about 50 ppma.
 7. The method of claim 5 wherein therecovered silicon particles have a content of metal contaminants of lessthan about 150 ppma.
 8. The method of claim 5 wherein the aqueous phasecomprises the silicon particles and abrasive particulate, and furtherwherein the aqueous phase is subjected to a density-dependent separationtechnique to separate the silicon particles from the abrasiveparticulate.
 9. The method of claim 8 wherein the density-dependentseparation technique is selected from sedimentation centrifugation,filtration centrifugation, and hydro-cyclone separation.
 10. The methodof claim 9 wherein, prior to the density-dependent separation, thesilicon particles are contacted with a source of iodine, in order toconvert at least a portion of the silicon present therein to SiI₄. 11.The method of claim 10 wherein, prior to the density-dependentseparation, the silicon particles are contacted with a liquid having adensity between about 2.25 and about 3.35 gm/cm³ to aid in theseparation of silicon and abrasive particulate present therein.
 12. Themethod of claim 11, wherein the silicon particles are contacted with anaqueous heteropolytungstate solution.
 13. The method of claim 5, whereinthe collected aqueous phase is dried prior to separating the siliconparticles from the abrasive particulate therein.
 14. The method of claim13, wherein the silicon particles are separated from the abrasiveparticulate after drying by subjecting the dried mixture to anon-uniform magnetic field.
 15. The method of claim 14, wherein themethod further comprises: creating a non-uniform magnetic field betweena first pole and a second pole of the magnet; aerosolizing the dried,solid particulate mixture; and feeding the aerosol into the non-uniformmagnetic field to separate silicon particles from abrasive grainspresent therein.